I see that the below command line works fine:
./KStars.app/Contents/MacOS/kstars test.fit
It opens up KStar with the image shown.

Releasing a separate version of KStars just to get this to work seems silly

On macOS the opening of files is related to the UTI of the file according to: en.wikipedia.org/wiki/Uniform_Type_Identifier#Looking_up_a_UTI
Below you can see that a file saved by KStars differs in the UTI from a file that is saved by another program: Siril. Note that the file name extensions also differ by the extra s in .fits in KStars vs .fit in Siril. Both are "FITS file" though.


[jd@Aok:Desktop]> mdls -name kMDItemContentType -name kMDItemContentTypeTree -name kMDItemKind test_saved_by_siril.fit
kMDItemContentType = "gov.nasa.gsfc.fits"
kMDItemContentTypeTree = (
"gov.nasa.gsfc.fits",
"public.image",
"public.data",
"public.item",
"public.content"
)
kMDItemKind = "FITS file"
[jd@Aok:Desktop]> mdls -name kMDItemContentType -name kMDItemContentTypeTree -name kMDItemKind test_saved_by_kstars.fits
kMDItemContentType = "dyn.ah62d4rv4ge80q4pysq"
kMDItemContentTypeTree = (
"public.item",
"dyn.ah62d4rv4ge80q4pysq",
"public.data"
)
kMDItemKind = "FITS file"

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